There are many studies on the fermentative production of vitamin B.sub.6. Various microorganisms belonging to the genera Saccharomyces [G. H. Scherr and M. E. Rafelson, J. Appl. Bacteriol. 25, 187-194 (1962)], Pichia [. Nishino, K. Fujii, and T. Kamikubo, Agric. Biol. Chem. 37, 553-559 (1973)], Klebsiella [R. Suzue and Y. Haruna, J. Vitaminol. 16, 154-159 (1970)], Achromobacter [M. Ishida and K. Shimura, Agric. Biol. Chem. 34, 327-334 (1970)], Bacillus [W. Pflug and F. Lingens, Hoppe-Seyler's Z. Physiol. Chem. 359, 559-570 (1978)] and Flavobacterium [Y. Tani, T. Nakamatsu, T. Izumi and K. Ogata, Agric. Biol. Chem. 36, 189-197 (1972)] are known to produce vitamin B.sub.6. But no commercially attractive fermentation process for the production of vitamin B.sub.6 has become known so far.
More recently, European Patent Publication 0 765 938 A2 describes a process for the fermentative production of vitamin B.sub.6 by cultivating a microorganism belonging to the genus Rhizobium capable of producing the vitamin in a culture medium under aerobic conditions. The culture medium may contain, apart from assimilable carbon and digestible nitrogen sources, inorganic salts and other nutrients and further substances which improve the vitamin B.sub.6 titer, such as DTP and HT. This process, however, is too inefficient.